Do you have any addional info as to number of transaction, type of data? Is this mission critical data, casual data any of 4 could be updating? Filemaker Pro offers several options.
Filemaker Pro 5 users - one computer running only Filemaker Pro with Filemaker filesharing for the four users.
Filemaker sells five user VLA for $1425
If the database is simple and use light, one copy of Filemaker ($300) allows five concurrent users accessing from a browser with IWP.
NOTE: there are limitations of what can be done with Instant Web Publishing.
Amazon lists Filemaker Pro 12 for $252 to $275 per seat.
Best would be a dedicated 64 bit computer serving the database with maximum RAM and a Solid State Drive. Users would have Filemaker.